Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device and a method of manufacturing the same, and more particularly, to an organic EL display device with an improved encapsulation structure, and a method of manufacturing the same.
Background of the Invention
Generally, organic EL display devices are self-luminescent display devices which emit light by electrically exciting a fluorescent organic compound. They can be operated with low voltage and made thin. In addition, organic EL display devices have advantages, such as a wide viewing angle and a fast response speed, that overcome some of the problems of liquid crystal displays. Accordingly, it is expected that organic EL display devices may be applied to next generation displays that can overcome some of the shortcomings of liquid crystal displays.
In such organic EL display devices, an organic layer is formed in a predetermined pattern on a substrate made of glass or other transparent insulators. Electrode layers are disposed above and below the organic layers. The organic layers are made of organic compounds.
In organic EL display devices having the above-described structure, when positive and negative voltages are applied to the electrodes, holes are moved from an electrode, to which a positive voltage is applied, to an emission layer via a hole transport layer. Electrons are moved from an electrode, to which a negative voltage is applied, to the emission layer via an electron transport layer. The electrons meet the holes in the emission layer, thereby generating excitons. The excitons make a transition from an excitation state to a ground state, thereby causing fluorescent molecules of the emission layer to emit light. As a result, an image is formed.
In the aforementioned organic EL display devices, moisture absorbed into the device may result in deterioration of device performance. Thus, organic EL display devices may require an encapsulation structure for preventing infiltration of moisture.
In conventional organic EL display devices, several encapsulation techniques have been proposed, including methods in which a metal can or a glass substrate is processed in the form of a cap having a groove to which a powdered absorbent for absorbing moisture is applied. Alternatively a film of the absorbent is adhered using a double-sided tape. However, since applying the absorbent to the groove may be quite complicated, the material and processing costs may increase, and the overall thickness of the substrate may increase. In the case of a powdered absorbent, it cannot be used for a front emission type display device since the substrate used for encapsulation is opaque. Meanwhile, in the case of a film-like absorbent, there is a limit to preventing the infiltration of moisture. Since the film-like absorbent may be easily damaged due to scratches or other external impacts during manufacture or use, high durability and reliability cannot be attained. Thus, the encapsulation structure using a film-like absorbent is not generally suitable for mass production of organic EL display devices.
Japanese Patent Laid-open Publication No. 9-148066 discloses an organic EL display device comprising a laminate in which organic luminescent material layers made of organic compounds are disposed between a pair of opposite electrodes and within a hermetic container for isolating the laminate from external air. A drying material is disposed in the hermetic container, wherein the drying material absorbs moisture so that the device maintains a solid state even as it absorbs moisture. Examples of the drying materials include alkali metal oxides, sulfuric salts and the like. Such an organic EL display device may become bulky due to the shape of a hermetic container. Also, even if the device is maintained at a solid state after absorption of the moisture, the device cannot be used for a front emission type display because of its opacity. Further, since the manufacturing process is complicated, the material cost and processing cost may increase.
Japanese Patent Laid-open Publication No. 5-335080 discloses a method of forming a protection film for an organic EL device. In the disclosed method, a shapeless silica protection film is formed on an organic EL device having an electroluminescent material layer containing at least one kind of organic compound between positive and negative electrodes, at least one of which is a transparent electrode. In the disclosed organic EL device, the structure for preventing moisture infiltration from the outside is configured such that dense, shapeless silica is thickly coated on one electrode. However, while the structure cannot absorb moisture inherently existing in the device, it can protect the organic EL device from external moisture. Thus, when using shapeless silica as a protective film material, separate structure for absorbing inherent moisture may be further necessary.